1) Field of the Invention
The present invention relates to a technology for reading a color image through a scanner to reproduce the image on a transfer paper in two colors of red and black.
2) Description of the Related Art
Multifunction peripherals that functions as printer, copier, facsimile, and scanner are known. Moreover, multifunction peripherals that form two-color images are also known.
Top row in FIG. 14 illustrates a pixel string with pixels A to G spaced at intervals of T and bottom row illustrates the same pixel string when the number of pixels in the pixel string is reduced to 80%. It is common to use the cubic convolution method to perform increase and/or reduction of pixels, because, this method is accurate. In this method, four consecutive pixels are selected from the pixel string as sampling pixels, and a virtual pixel that is a pixel after increase and/or reduction of pixels is obtained by using a cubic function as a distance parameter.
If the pixels that are at interval T in the pixel string shown in FIG. 14 are to be reduced to 80%, the interval between two virtual pixels will be 100/80=1.25T. For example, a virtual pixel α is obtained by using a cubic function G (x) (not shown) and four pixels A, B, C, and D as sampling pixels.
FIG. 15 illustrates how data is generated in varying processing in the apparatus that reproduces an image in two colors. The original color data is converted into binary color data, and the original density data is converted into binary density data. However, if an unbalance is generated between the original color data and the binary color data or the original density data or the binary density data, the desired image is not produced after the varying processing.
Suppose the cubic function convolution method is used for varying processing of the density data and simple interpolation, which does not take into consideration the phase component, is used for varying processing of color data. The density data at a virtual pixel (B) between the sampling pixel (A) as black and a sampling pixel as any other color each identified by a color identifying unit is calculated using information for four pixels forward and backward from the virtual pixel B. On the other hand, the color data is calculated using information for one forward pixel.
Therefore, a black pixel “a” is generated unlike the original image. This represents a shift between the color data and the density data.